1. Field of the Invention
The present invention generally relates to a portable device for regulated production of heat, and more particularly to a portable heat generating device in which heat is uniformly generated across the surface of a sheet-shaped, fabric-like structure.
2. Description of the Prior Art
Several portable chemical heat generating devices are known which are incorporated into, for example, outerwear, garments, blankets and sleeping bags.
A first type of device is taught in U.S. Pat. No. 4,516,564 and U.S. Pat. No. 4,756,299. This first type of device includes a powdered, exothermic material, such as oxidizable metal, which is maintained in a sheet-like form and covered with a porous, air permeable sheet. The amount of air permeating the sheet is regulated to control the reaction rate of the exothermic materials, thereby controlling the amount of generated heat.
A second device is taught in U.S. Pat. No. 5,425,975. In this second device, exothermic material is dispersed in and supported by a sheet-like substrate made up of a plurality of irregularly arranged fibers having a multiplicity of gaps therebetween which facilitate air flow to the exothermic material. The sheet-like substrate is held in a bag having air-permeation holes. As with the first type of device, the amount of air entering the sheet-like substrate passing through the gaps is controlled such that the exothermic material generates a desired amount of heat.
A third device is taught in U.S. Pat. No. 5,125,392. In this device, exothermic material is held within a multitude of holes formed in a thermogenic material mat located between a pair of panels. Air is supplied to the exothermic material by a pump through a first plurality of air passages, and exhaust gases exit though a second plurality of air passages. The amount of heat generated by the exothermic material is controlled by controlling the air flow through the pump.
A problem associated with the above-mentioned first, second and third known device types is that the exothermic material is depleted after a period of use, thereby terminating the heat generating process. When the exothermic material is depleted, it is necessary to either dispose of some or all of the heating device, or to perform a cumbersome and time consuming process of replacing or regenerating the exothermic material. These characteristics make such devices impractical for multi-day travel on foot in isolated geographic locations where weight, convenience and refuse considerations are important.
Another problem associated with the above-mentioned first and second device types is that heat production is turned on and off relatively slowly because it is regulated by means of natural diffusion of air through permeable membranes of large surface area. Further, if these devices are used for warming parts of the body other than the extremities, turning these devices off requires physical removal of the devices from the body and storage in an air tight compartment. Because these heating devices are usually worn under a passive outer garment in these instances, they are not well suited for heat-on-demand applications where it is impractical or inconvenient to remove the outer layers of clothing.
Still other important factors reduce the usefulness of the above-mentioned prior art portable heat generating devices. For instance, the use of oxidizable powdered metals for generating heat at ambient temperatures will generally require a controlled and continuous supply of air (i.e. oxidizer) to produce and regulate heat output. The continuous flow is necessary to sustain a high reaction rate sufficient to generate adequate heat. Additionally, the rate of air flow must be sufficient to avoid non-uniform distribution of heat, as described in U.S. Pat. No. 5,125,392, mentioned above. Thus, significant energy must be expended to drive an air pump in a constant and dependable manner.
A fourth portable heat generating device is taught in U.S. Pat. No. 4,685,442. This portable heating device generates heat in a heat exchanger with is mounted at a location remote from the desired point of application of the heat. A circulating heat transfer fluid is pumped through the heat exchanger and then delivered to a desired point to perform the warming function. However, because of heat loss from the heat transfer fluid while it travels to the desired point, the energy efficiency of this device is poor. Furthermore, the device is relatively heavy because, in addition to the fuel required to provide the heat energy, the heat transfer liquid is required to transport the heat to the desired point. Another shortcoming of the fourth device is that the extra bulk and structural rigidity associated with a heat exchanger can reduce wearing comfort when worn for long periods. These and other limitations associated with the chemical heat generating devices, of which representative examples are described above, appreciably reduce their marketability for use in, for example, outerwear, garments, blankets and sleeping bags.